1. Field of the Invention
The present invention relates to power amplifiers, and specifically to a power amplifier circuit including a predistortion amplifier that is of the same type as amplifiers in an amplifier array and that improves the linearity of a signal amplified by the array.
2. Description of Related Art
High power amplifier circuits used to amplify modulated signals in, for example, wireless communications base stations often are designed to create a certain amount of predistortion to compensate for distortion generated by the signal amplifier. Typically, an input modulated signal will be split into two signal components, with one signal component being conditioned to match input power requirements of the signal amplifier, and the other signal component being predistorted before being re-combined with the first component. When the resulting re-combined signal is input into the signal amplifier, the predistortion helps to compensate for nonlinearities added to the resulting amplified signal due to, for example nonlinear transconductance (if a FET device is used) and junction capacitance and the saturation effects of the amplifier that result in signal clipping.
Conventionally, devices such as diodes are used to predistort the above-discussed first signal component as such devices are efficient and therefore use minimal power. However, it is difficult at best to match the predistortion with the amplifier-generated distortion because the predistortion and distortion are generated by dissimilar component characteristics and nonlinear mechanisms. To generate a more accurate distortion match, feedforward amplifier techniques can be employed, where expensive error amplifiers must be implemented to process the signal amplifier distortion.
Digital predistortion may alternatively be used in place of the above analog predistortion. However, the additional components necessary for signal feedback as well as the resulting power dissipation increase when compared to analog predistortion solutions, and the characteristic limited bandwidth of correction make such an option impractical in most circuit design situations that have nonlinearities higher than fifth order.